1. Field of the Invention
This invention relates to a process for the continuous electrolytic coloring of an article of aluminum or an aluminum base alloy (hereinafter referred to aluminum) in a windable shape, such as an aluminum strip or wire.
2. Description of the Prior Art
Various processes for the electrolytic coloring of aluminum comprising subjecting aluminum which has previously been anodically oxidized to electrolysis in an electrolytic coloring bath comprising an aqueous solution of at least one nickel salt, cobalt salt, tin salt, iron salt, copper salt or selenious acid, the aluminum serving as an electrode, are well known in the art. These electrolytic coloring processes are known to be economically advantageous as they enable one to produce electrolytically colored aluminum articles with excellent reproducibility which are uniform in color. In addition, because of their excellent weatherability or fade resistance, the resulting electrolytically colored aluminum generally can be used as structural materials such as extrusions, plates and the like.
Known electrolytic coloring processes can be classified as alternating current electrolysis processes as disclosed in U.S. Pat. No. 3,382,160 or direct current electrolysis processes as disclosed in U.S. Pat. No. 3,761,362, depending upon the kind of electric current which is passed through the electrolytic coloring bath.
The process disclosed in U.S. Pat. No. 3,761,362 is characterized by the coloration of a previously anodized aluminum by subjecting the aluminum to a direct current electrolysis, with the aluminum as a cathode, in an electrolytic coloring bath comprising an aqueous solution containing at least one nickel salt, cobalt salt, tin salt, iron salt, copper salt or selenious acid.
In this direct current electrolysis, the composition of the electrolytic coloring bath is suitably selected from the above-described components depending on the desired color. Generally, the color formed on the surface of the aluminum is bronze when a nickel salt is used, reddish-brown when a copper salt is used, bronze to black when a tin salt is used, bronze when a cobalt salt is used, yellow when an iron salt is used, and yellow to reddish-orange when selenious acid is used.
The above direct current electrolytic coloring process can be carried out in a continuous manner which is advantageous in that electrolytically colored aluminum can be obtained economically by continuously anodically oxidizing a windable aluminum article such as strips, wires and the like, and then subjecting the resulting anodically oxidized aluminum article to a direct current electrolytic coloring process, followed by conventional final treatments such as a sealing treatment or various coating treatments. However, when the electrolytic coloring process is effected in a conventional electrolytic cell containing an electrolytic coloring bath by continuously supplying aluminum as a cathode from one end of the electrolytic coloring cell which is provided with an anode on an inside wall or on the bottom of the cell and with agitation means for the coloring bath, e.g., by injecting air or the coloring bath into the cell, it is very difficult to obtain a uniformly colored aluminum in a stable manner. That is, satisfactory coloring is difficult to conduct due to the problems hereinafter described, whereby the resulting color tone tends to be non-uniform, for example, a stripe-pattern comprising a deeply-colored portion and an unattractive lightly-colored portion results, and the electrolytically colored film on the aluminum tends to be partially spalled. These phenomena are frequently observed with an electrolytic coloring process conducted on a large industrial scale, and are liable to occur as the travelling speed of the aluminum passing through the coloring bath increases and the electric current applied to the coloring bath increases.